This invention relates to a vector comprising a satellite RNA of a plant virus.
It is known that so-called satellites have relatively low molecular weight (e.g., 0.1-0.5.times.10.sup.6 dalton) and barely have homology with the genome RNA of the helper virus (parent virus).
So-called satellites of plant viruses are classified into two categories.
One category is satellite viruses wherein satellite RNA is coated with the coat protein which is encoded by satellite RNA, for example, satellite tobacco necrosis virus (STNV), satellite panicum mosaic virus (SPMV) and satellite tobacco mosaic virus (STMV).
The other category is satellite RNAs contained in helper virus particles, a large number of low molecular weight RNAs belong thereto, typical examples are cucumber mosaic virus (CMV) satellite RNA, tobacco ring spot virus (TobRV) satellite RNA, etc. Research on the molecular level is being vigorously carried out.
More particularly, base sequences of several strains have been determined in CMV satellite RNA and reported. For example, Hidaka et al.: FEBS Letters 174, p38-42 (1984); Garcia-Arenal et al.: Virology 158, p339-347 (1987).
Further, there has been a successful insertion of the full length cDNA of CMV satellite RNA into a DNA transcription vector and the creation of an infectious transcription product with RNA polymerase in a test tube (Collmer & Kaper: BBRC 135, p290-296 (1986); Masuta et al.: Nucl. Acids. Res. 15, p10048 (1987); Kurath & Palukaitis: Virology 159, p199-208 (1987)).
Incidentally, there have been two cases of practical trials to introduce an exogenous RNA fragment in plant cells by changing a plant RNA virus into a vector: brome mosaic virus (BMV) RNA 3 (French et al.: Science 231, p1294-1297 (1986)) and tobacco mosaic virus (TMV) RNA (Takamatsu et al.: EMBO J.6, p307-311 (1987)). In each case a recombinant transcription vector was created wherein the coat protein gene region is replaced by chloramphenicol acetyltransferase (CAT) gene.
However, in the former case, utilization of the recombinant transcription vector was limited to inoculation of a protoplast and it was impossible to infect barley as a whole plant.
Further, in the latter case, although inoculation of the recombinant transcription vector into tobacco was successful, it was found that the vector did not systemically move from the inoculated leaf.
Further, in both cases, replication ability was lowered to 10.sup.-2 to 10.sup.-3 compared to the natural virus.
The above described problems become practically fatal drawbacks when plants are transformed using the RNA vectors or when useful proteins are produced using the vectors.
Thus, it is necessary to satisfy the following points for using plant RNA viruses as a vector:
(1) Such a virus has a high replication ability in plant cells;
(2) When an exogenous DNA fragment is inserted into the virus nucleic acid, the virus can maintain biological activity;
(3) It is desirable that the virus is systemically infectious; and
(4) The virus has a host range as wide as possible.